1. Field of the Invention
This invention relates to silicon production and more particularly to processes and devices for producing high purity silicon. Specifically, this invention relates to a process and apparatus for refining impure silicon, such as metallurgical grade silicon, to form high purity silicon usable in photovoltaic solar cells, semiconductor devices and the like.
2. Description of the Prior Art
High purity silicon is both useful and oftentimes required for a wide variety of industrial applications. One such application is in the area of photovoltaic solar cells. In such photovoltaic cells, thin sheets or wafers of highly refined silicon form at least the upper surface of a multi-layered cell adapted for direct conversion of incident solar radiation to an electrical potential. To date, the processes and techniques available to produce high purity silicon and to subsequently form such silicon into thin sheets or wafers are extremely expensive.
One commonly used technique for electrochemically purifying silicon is an adaptation of a well-known aluminum refining process. In this technique, silicon is substituted for aluminum to provide an electrochemical process utilizing a molten Cu/Si anode with Na-based based molten electrolyte. The electrolyte contains Na.sub.3 AlF.sub.6 for transporting silicon to a cathode. An example of such a process is disclosed in the French paper by R. Monnier and J. C. Giacometti entitled, "Recherches sur la Raffinage Electrolytique du Silicium," Helvetica Chimica Acta, Vol. 47,345, (1964).
There are several significant problems with the above described type of system. One such problem is that since the anode must be molten, only a single electrode pair per electrochemical cell can be utilized. Therefore, silicon deposition per unit cell volume on the cathode proceeds rather slowly. Furthermore, due to the dynamics of such cells, large electrolyte vapor losses result from the high cell operating temperatures. Consequently, this particular system is not very economical.
Another known electrochemical process for plating Si is described in a paper by Uri Cohen entitled, "Some Prospective Applications of Silicon Electrodeposition from Molten Fluorides to Solar Cell Fabrication," J. Electronic Mat'ls., Vol. 6, #6, 607 (1977). In this particular process, a LiF, KF, K.sub.2 SiF.sub.6 molten salt electrolyte is used to plate silicon onto a graphite cathode. However, a substantially pure solid sheet Si anode is required. Thus, this technique has no practical application for use with metallurgical grade or other impure silicon.
Still another electrochemical process is disclosed in U.S. patent application Ser. No. 387,115, filed June 10, 1982, and assigned to the assignee of the present invention. In this process, a Cu/Si anode is used as a silicon source for electrochemical deposition of Si onto a graphite cathode in a molten salt electrolyte.
Other techniques presently being utilized to produce highly pure silicon for the semiconductor and photovoltaic industries include well-known distillation processes as well as various chemical conversion techniques. One such common technique includes the conversion of metallurgical grade silicon to a basic intermediate trichlorosilane (HSiCl.sub.3) via a fluidized bed reaction with anhydrous hydrogen chloride as indicated in reaction equation 1 below. EQU Si+3HCl=HSiCl.sub.3 +H.sub.2 ( 1)
This is followed by the purification of the trichlorosilane by distillation and then a subsequent deposition of semiconductor grade silicon via chemical vapor deposition from trichlorosilane in the presence of hydrogen as indicated by reaction equation 2 below. EQU HSiCl.sub.3 +H.sub.2 =Si+3HCl (2)
Typically, the deposition of silicon from trichlorosilane in the presence of hydrogen may take place in a Siemens-type reactor. However, the Siemens reactor is not complete in that unreacted trichlorosilane and H.sub.2 as well as SiCl.sub.4 are produced in addition to the silicon and hydrogen chloride. The above technique is thoroughly disclosed in a Dow Corning report entitled, "Polysilicon Technology," by Leon D. Crossman and John A. Baker, which report also appeared in "Semiconductor Silicon 1977," H. R. Huff and E. Cirtl, editors, The Electrochemical Society Softbound Symposium Series, Princeton, N. J. (1977). This technique is very expensive to operate in order to produce sufficient quantities of highly pure or refined silicon due to the inefficiency of this Siemens-type reactor as well as due to the numerous different process stages required in this overall technique.
One of the major problems and hurdles facing the photovoltaic industry is in reducing the costs associated with producing photovoltaic cells. Efficient photovoltaic cells utilizing silicon are, in fact, presently available and could readily be utilized on a mass basis if it were not for a prohibitive pricing structure. An important aspect of this pricing structure is the direct result of the high cost of purifying silicon and forming such purified silicon into thin wafers or sheets. The present invention meets both these needs of the photovoltaic industry, as well as the related semiconductor industry, by providing highly refined silicon at an economical cost as well as providing such highly refined silicon potentially in sheet form at no substantial additional cost.